A wireless communication technique has been used worldwide in which its communication method is standardized as a standard for each of applications, such as radio broadcasting, TV broadcasting, narrow band land radio, mobile phones, and wireless LAN (Local Area Network). The government and regulatory organization of each country performs radio frequency allocation to each application, system, and provider while considering the radio propagation characteristic different according to frequency.
Conventionally, in many wireless systems, a single frequency band is allocated to be divided into a plurality of channels for operation. In mobile phones, to accommodate more users, a plurality of frequency bands are allocated to perform area design according to operation policy. For example, there is an idea of allocating a frequency below 1 GHz to achieve non-line-of-sight communication by the diffraction effect and of using a high frequency near 2 GHz for operating a relatively small cell.
In addition, to accommodate increasing traffic, traffic offloading that performs data communication by wireless LAN not via a cellular network has been widely used in recent years. Further, attention has been drawn to a technique called HetNet in which systems having different cell radii (of a macro cell and a small cell) and methods are mixed in the same area and operate cooperatively to improve the system capacity. In the HetNet, proposed is the application of a C/U split (control plane/user plane split) technique that allows the small cell to perform only data communication and allows the macro cell to perform cell control (such as terminal control) (for example, see Patent Literature 1 and Nonpatent Literature 1).
Typically, when high speed communication is achieved, wide bandwidth transmission is required. For example, the bandwidth of 20/40/80/160 MHz is used for wireless LAN (WiFi, IEEE 802.11ac), the bandwidth of 1.4/3/5/10/15/20 MHz is used for LTE (Long Term Evolution), and the bandwidth of 2.16 GHz is used for WiGig using millimeter waves (IEEE 802.11ad). In contrast, the transmission power is limited according to inter-cell interference design and use conditions (mobile station/fixed station, and communication distance). In particular, the transmission power of the mobile station is often in the range of several mW to several hundreds mW from the viewpoint of size reduction and the life of the battery.
When the bandwidth is widened under the conditions that the transmission power is constant, the transmission power per 1 Hz (power spectral density; PSD) becomes smaller to deteriorate the signal-to-noise power ratio (SNR). This worsens the communication quality. In addition, the receiving power becomes smaller with distance. Thus, as the bandwidth is wider, long distance communication becomes more difficult.
Moreover, in the recent frequency use conditions, low frequency is overcrowded, and higher frequency can secure a wide frequency bandwidth more easily. From this, the novel system that tries to achieve high speed communication tends to use high frequency. However, since higher frequency tends to increase the propagation loss, the service to a wide area including long distance communication and non-line-of-sight communication is difficult.